Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter II. Histology
2.5. Vascular Tissues

They have a specific Structure and various adaptations for the rapid Transport of Assimilates from the Organs where they are produced or absorbed to the sites of utilization or storage, as well as for the Movement of Water and dissolved mineral salts from the soil to the stem and leaves.

Accordingly, two main streams of substances have formed in the plant Organism during evolution: upward and downward. Water with dissolved Mineral Substances moves upward via the upward stream—that is, from the roots through the stem to the leaves—while organic nutrients synthesized in the leaves move downward from the leaves to the roots or to the sites of utilization and storage.

Water and dissolved mineral salts are transported through specialized histological elements: tracheids and tracheae, or vessels. Tracheids are elongated, closed Cells with obliquely truncated, pointed, or rounded ends. Their longitudinal walls are unevenly thickened. Each tracheid communicates with adjacent ones via bordered pits. Bordered pits differ from simple ones in that the pit cavity narrows from the closing membrane toward the inner pit aperture. The walls of tracheids are unevenly thickened, being adapted to perform two Functions: The transport of aqueous solutions and the mechanical reinforcement of the organ.

Tracheae, or vessels, are tubes consisting of a vertical row of cells known as vessel elements, with openings called perforations located in the partition walls between them. Tracheae develop from a vertical row of meristematic cells that progressively elongate; their longitudinal walls become impregnated with Lignin, thickened, and lignified, the protoplast dies, and the cross-walls between the elements dissolve completely or partially.

Based on the pattern of wall thickening, vessels and tracheids can be annular, spiral, annular-spiral, scalariform, or pitted (Fig. 23). The more advanced conducting Tissues are scalariform and pitted vessels, which are characteristic mostly of flowering plants.

Vessels and tracheids, together with xylem parenchyma and other histological elements, form the xylem, or wood.

Tracheids function for 1–5 years and subsequently become occluded by tyloses, mucus, Gums, or mineral substances, thus losing their water-conducting function.

Tyloses are outgrowths of the parenchyma cells surrounding a vessel that bulge into its lumen.

In the course of evolution, tracheids appeared first; they are part of the xylem in lower vascular plants and most gymnosperms, whereas both tracheids and vessels occur in angiosperms and highly evolved gymnosperms.

Plastic substances are transported from the leaves to the roots through sieve tubes and companion cells. Sieve tubes are a vertical row of living, elongated cells characterized by the presence of sieve-like transverse plates that facilitate a uniform flow of assimilates. Sieve plates are thin partitions pierced by numerous pores traversed by plasmodesmata. Typically, sieve plates are located in transverse or oblique end-walls, though they are occasionally found on lateral walls as well, which is characteristic of gymnosperms. In most plants, sieve tubes are accompanied by companion cells derived from mother cells. Division of the mother cell produces two cells: one develops into a sieve tube element, while the other becomes its companion cell, with The Nucleus remaining in the companion cell. Sieve tubes and companion cells, together with phloem parenchyma, constitute the phloem. Sieve tubes function for a single growing season and are subsequently blocked by a substance called callose—callus—whereupon the tube loses its conducting function. Only in linden and grapevine does the callose dissolve in spring, allowing the tube to function for another 2—3 years.